Telecom Crash Course.pdf

Source: Telecom Crash Course

CHAPTER

First Things

First

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First Things First

Chapter 1

Telecommunications, like all highly visible and interesting fields, is full

of apocryphal stories, technical myths, and fascinating legends. Everyone

in the field seems to know someone who knows the outside plant repair

person who found the poisonous snake in the equipment box in the man-

hole 1 , the person who was on the cable-laying ship when they pulled up

the cable that had been bitten through by some species of deep water

shark, some collection of seriously evil hackers, or the backhoe driver

who cut the cable that put Los Angeles off the air for 12 hours.

There is also a collection of techno-jargon that pervades the tele-

commnications industry and often gets in the way of the relatively

straightforward task of learning how all this stuff actually works.

To ensure that such things don’t get in the way of absorbing what’s in

this book, I’d like to begin with a discussion of some of them.

This is a book about telecommunications, which is the science of com-

municating over distance (tele-, from the Greek tele, “far off ”). It is, how-

ever, fundamentally dependent upon data communications, the science

of moving traffic between computing devices so that the traffic can be

manipulated in some way to make it useful. Data, in and of itself, is not

particularly useful, consisting as it does of a stream of ones and zeroes

that is only meaningful to the computing device that will receive and

manipulate those ones and zeroes. The data does not really become use-

ful until it is converted by some application into information , because a

human can generally understand information. The human then acts

upon the information using a series of intuitive processes that further

convert the information into knowledge , at which point it becomes truly

useful. Here’s an example: A computer generates a steady stream of ones

and zeroes in response to a series of business activities involving the

computer that generates the ones and zeroes. Those ones and zeroes are

fed into another computer, where an application converts them into a

spreadsheet of sales figures (information) for the store from which they

originated. A financial analyst studies the spreadsheet, calculates a few

ratios, examines some historical data (including not only sales numbers

but demographics, weather patterns, and political trends), and makes an

informed prediction about future stocking requirements and advertising

focal points for the store based on the knowledge that the analyst was

able to create from the distilled information.

Data communications rely on a carefully designed set of rules that

governs the manner in which computers exchange data. These rules are

called protocols , and they are centrally important to the study of data

1 I realize that this term has fallen out of favor today, but I use it here for historical accuracy.

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First Things First

communications. Dictionaries define protocol as “a code of correct con-

duct.” From the perspective of data communications, they define it as “a

standard procedure for regulating the transmission of data between com-

puters,” which is itself “a code of correct conduct.” These protocols, which

will be discussed in detail later in this book, provide a widely accepted

methodology for everything from the pin assignments on physical con-

nectors to the sublime encoding techniques used in secure transmission

systems. Simply put, they represent the many rule sets that govern the

game. Many countries play football, for example, but the rules are all

slightly different. In the United States, players are required to weigh

more than a car, yet be able to run faster than one. In Australian Rules

football, the game is declared forfeit if it fails to produce at least one body

part amputation on the field or if at least one player doesn’t eat another.

They are both football, however. In data communications, the problem is

similar; there are many protocols out there that accomplish the same

thing. Data, for example, can be transmitted from one side of the world

to the other in a variety of ways including T1, E1, microwave, optical

fiber, satellite, coaxial cable, and even through the water. The end result

is identical: the data arrives at its intended destination. Different proto-

cols, however, govern the process in each case.

A discussion of protocols would be incomplete without a simultaneous

discussion of standards . If protocols are the various sets of rules by which

the game is played, standards govern which set of rules will be applied

for a particular game. For example, let’s assume that we need to move

traffic between a PC and a printer. We agree that in order for the PC to

be able to transmit a printable file to the printer, both sides must agree

on a common representation for the zeroes and ones that make up the

transmitted data. They agree, for example (and this is only an example)

that they will both rely on a protocol that represents a zero as the

absence of voltage and a one as the presence of a three-volt pulse on the

line, as shown in Figure 1-1. Because they agree on the representation,

the printer knows when the PC is sending a one and when the PC is

sending a zero. Imagine what would happen if they failed to agree on

such a simple thing beforehand. If the transmitting PC decides to repre-

sent a one as a 300-volt pulse and the printer is expecting a three-volt

Figure 1-1

Voltage

representations

of data.

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First Things First

Chapter 1

pulse, the two devices will have a brief (but inspired) conversation, the

ultimate result of which will be the release of a small puff of silicon

smoke from the printer.

Now they have to decide on a standard that they will use for actually

originating and terminating the data that they will exchange. They are

connected by a cable (see Figure 1-2) that has nine pins on one end and

nine jacks on the other. Logically, the internal wiring of the cable would

look like Figure 1-3. However, when we stop to think about it, this one-

to-one correspondence of pin-to-socket will not work. If the PC transmits

on pin 2, which in our example is identified as the send data lead, it will

arrive at the printer on pin 2—the send data lead. This would be analo-

gous to holding two telephone handsets together so that two communi-

cating parties can talk. It won’t work without a great deal of hollering.

Instead, some agreement has to be forged to ensure that the traffic

placed on the send-data lead somehow arrives on the receive data lead

and vice versa. Similarly, the other leads must be able to convey infor-

mation to the other end so that normal transmission can be started and

stopped. For example, if the printer is ready to receive the print file, it

might put voltage on the data terminal ready (DTR) lead, which signals

to the PC that it is ready to receive traffic. The PC might respond by set-

ting its own DTR lead high as a form of acknowledgment, followed by

Figure 1-2

assignments

on a cable

connector.

Send Data

(pin 2)

Ground

Receive Data

Carrier

Request to Send

Data Set Ready

Clear to Send

Data Terminal Ready

Figure 1-3

Logical wiring

scheme.

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First Things First

transmission of the file that is to be printed. The printer will keep its

DTR lead high until it wants the PC to stop sending. For example, if the

printer senses that it is running out of buffer space because the PC is

transmitting faster than the slower printer can print, it will drop the

DTR lead, causing the PC to temporarily halt its transmission of the

print file. As soon as the printer is ready to receive again, it sets the DTR

lead high once again, and printing resumes. As long as both the trans-

mitter and the receiver abide by this standard set of rules, data commu-

nications will work properly. This process of swapping the data on the

various leads of a cable, incidentally, is done by the modem—or by a null

modem cable that makes the communicating devices think they are talk-

ing to a modem. The null modem cable is wired so that the send-data

lead on one end is connected to the receive data lead on the other end and

vice-versa; similarly, a number of control leads such as the carrier detect

lead, the DTR lead, and the data set ready (DSR) leads are wired

together so that they give false indications to each other to indicate that

they are ready to proceed with the transmission, when in fact no

response from the far end modem has been received.

Standards: Where Do They

Come From?

Physicists, electrical engineers, and computer scientists generally design

data communications protocols. For example, the Transmission Control

Protocol (TCP) and the Internet Protocol (IP) were written during the

heady days of the Internet back in the 1960s by such early pioneers as

Vinton Cerf and the late John Postel. (I want to say “back in the last cen-

tury” to make them seem like real pioneers.) Standards, on the other

hand, are created as the result of a consensus-building process that can

take years to complete. By design, standards must meet the require-

ments of the entire data and telecommunications industry, which is of

course global. It makes sense, therefore, that some international body is

responsible for overseeing the creation of international standards. One

such body is the United Nations. Its 150 plus member nations work

together in an attempt to harmonize whatever differences they have at

various levels of interaction, one of which is international telecommuni-

cations. The International Telecommunications Union (ITU), a sub-

organization of the UN, is responsible for not only coordinating the

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